Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F52: Surface Chemistry and Photomaterials |
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Sponsoring Units: DCMP Chair: Emily Bittle, National Institute of Standards and Technology Room: Mile High Ballroom 1E |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F52.00001: Work Function Studies of Carbon Nanospikes for Electrochemistry by UPS Arthur Baddorf, Yang Song, Dale K. Hensley, Adam J. Rondinone Carbon nanospikes show excellent energy and selectivity for electrochemical conversion of CO2 to ethanol1 and N2 to NH3.2 Reactivity is attributed to physical structure more than composition. A spike geometry concentrates the electric field, promoting local electroreduction of reaction molecules. Using UPS, we have measured the work function of carbon nanospikes transferred to vacuum after extraction from a N2-saturated LiClO4 electrolyte at a range of potentials. The work function exhibits a general decrease with increasing negative emersion potential. The decrease is not 1:1 eV/V to the applied potential, nor is it linear. This may reflect imperfect double layer retention during transfer through air and into vacuum. In most cases, UPS spectra were typical, with an obvious low-energy cutoff. However, for several samples, including the pristine nanospikes, two cutoffs appeared to exist, one suggesting a much lower work function. The nature of the low-energy cutoff and possible connection to the geometry will be discussed. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F52.00002: Structural and Electrical Factors of the Photocatalytic Activity of Bi2WO6 and Bi2MoO6 in Water-Splitting Applications Steven Baksa, Quinn Campbell, Ismaila Dabo Artificial photosynthesis is a sustainable technological option to store solar energy through the photocatalytic conversion of carbon dioxide and water into chemical fuels. The main challenges with this photocatalytic process include tuning the band gap of the material to match the solar spectrum and ensuring its stability in aqueous environments. An appealing approach for developing such photocatalysts consists of intercalating functional layers into metal oxides, as is the case for perovskite-derived compounds of the Aurivillius (Bi2An-1BnO3n+3) family, which have been shown to be photoactive under visible light. Using density-functional theory, we predict the nanosheet morphology of Bi2WO6 and Bi2MoO6 under applied voltage, and we provide a molecular description of the charged interface under controlled pH and applied voltage. This study offers a molecular interpretation of the competing structural and electrical factors that underlie the facet-dependent photocatalytic activity of layered Bi2An-1BnO3n+3 compounds. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F52.00003: Density Functional Theory Studies of Protective Interlayer of Graphene for the Back Contact of Copper Zinc Tin Sulphur Thin Film Solar Cell Yau Lun Felix Chong, Junyi Zhu CZTS(Se) based solar cell is one of the promising candidates in the development of the 3rd generation solar cell absorbers. However, the efficiency of CZTS is insufficient for commercial product. One of the major causes of low performance is poor back contact-absorber interface quality. During annealing process of CZTS, S atom diffuses into Mo back contact layer and form a detrimental secondary phase of MoS2. Interlayer engineering has been proposed. However, most of the proposed interlayers are based on insulating systems. Here, we propose a new protective material of graphene as an effective diffusion barrier to block any S atom diffusion. Using Climbing Image Nudged Elastic Band Method based on Density Functional Theory calculation to find diffusion barrier and minimum energy path, we can show that the choice of our protective layer can effectively block S atoms diffusion under fabrication and operation environment. We also calculated the band alignment between the proposed protective interlayer and CZTS absorption layer. The results suggest the protective interlayer we proposed will not significantly degrade the performance of the solar cell. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F52.00004: The Electromagnetic Enhancement of SERS and the Modified Partition of Optical States in the Strong Matter-Coupling Regime. Kritika Jain, Murugesan Venkatapathi Surface enhanced Raman Spectroscopy (SERS) is a powerful optical sensing technique that is based on enhanced Raman signals from molecules in proximity of rough metal surfaces. Experiments1 have shown unexpected large enhancements in SERS, even up to 1014. Conventional electromagnetic theory accounts for enhancements only up to 106, and the anomalous enhancements have even been attributed to an unknown chemical origin2,3. We show that when one includes dynamics in fluctuations of an emitter strongly coupled to absorbing matter, such high gains are predicted due to tunneling out of photons from the strongly absorbing metal surface. This modification to the conventional partition of optical states into its radiative and non-radiative parts, is also imperative for emitters proximal to limiting small metal nanoparticles (< 10 nm in dimensions) which are fully absorbing. Some recent experiments have shown such anomalous gains in emission due to these extremely small nanoparticles. This effect can be exploited further in light generation, optical sensing and radiative heat transfer. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F52.00005: Protecting high quantum efficiency photocathodes with two dimensional materials for long lifetimes Gaoxue Wang, Nathan A Moody, Ping Yang, Enrique Batista Since soon after the discovery of the photoelectric effect, the goal of reaching highly efficient photoelectron emission has been a difficult and well sought out goal. Currently, applications such as XFEL and XERL, call for high intensity electron sources that can lead to bright and intense electron and x-ray beams. The intensity of the photoemitted beam, the quantum efficiency of the photocathode, is dominated by the absorption cross section, electron mobility, and surface work function. For practical reasons, the lifetime of the photocathode is important and long-lived photocathodes are very much sought to improve performance. Whereas alkali-based semiconducting photocathodes display much higher QE relative to metal surfaces, their chemical composition makes them very reactive to residual gases even at UHV conditions. We will discuss here approaches for neutralizing such reactivity on the surface not introducing higher barriers to the emitted electrons. A computational study will be presented for screening 2-dimensional coating materials and fundamental principles for the characteristics needed are extracted. We present not only design principles but also candidate materials that should be used for protecting these highly reactive surfaces |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F52.00006: Defect mediated self-powered, broad band and ultrafast InGaN based photodetector ARUN CHOWDHURY, Rohit Pant, Basanta Roul, Deependra Kumar Singh, Karuna Kar Nanda, Saluru Baba Krupanidhi In this work, Si doped n+-InGaN epilayer has been grown on a 100 nm thick AlN template on an n-type Si (111) substrate to form semiconductor-insulator-semiconductor (SIS) heterostructure by plasma-assisted molecular beam epitaxy (PAMBE). The n+-InGaN/AlN/n-Si (111) device shows excellent self-powered and broad band photo response under UV-Visible (300-800 nm) light illumination and maximum response is observed at 580 nm for low intensity irradiance (0.1 mW/cm2), owing to the intermediate energy states present in InGaN lattice due to nitrogen vacancies. At zero bias, the device exhibits a high responsivity of 9.64 A/W with ultrafast rise and fall times of 19.9 and 21.4 μs, respectively. This is the highest reported responsivity for the InGaN based photodetectors at zero bias to best of our knowledge. Introduction of AlN buffer layer and doping enhance the photoelectrical properties of the device compared to other conventional detectors. This work opens up a new avenue for SIS heterojunction photo detectors with much improved performance as self-powered and broadband detectors over the previously reported values on InGaN. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F52.00007: The Effect of Au Nanoparticles on the Photovoltaic Conversion Efficiency in CdTe/CdS Thin Films Yunis Yilmaz, Noelle Schilling, Stephane Arsharuni, Mehmet Alper Sahiner In our last study, we have worked on the effects Ag and Au nanoparticles have on the efficiency of CdS/CdTe thin film solar cells. Both had a positive effect on the efficiency of photovoltaic conversion. Now, we are testing Au nanoparticles on the same cells and plan on comparing the effects of different deposition times, and laser energy on the Au, and how it effects efficiency and Au nanoparticle size. We used PLD (pulsed laser deposition) to create these cells. The laser shoots the target material we want to deposit (CdS,Au,CdTe) turning it into plasma plume that deposits onto ITO (indium tin oxide) coated glass. Varying how long the laser shoots at the Au and at what energy changes the amount of Au deposited and the size and distribution of the particles as well. The more Au in the np junction, the more the increase in the efficiency of the solar cell due to plasmonic resonance effects. However, the efficiency cuts off with after a threshold value for Au deposition. Our last study concluded that we had already passed this saturation point, so now, we are testing lower deposition lengths and energies. The Au embedded cells were characterized by XRD, AFM, SEM/EDX, and a sourcemeter setup. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F52.00008: Development of Molybdenum Nitride Thin-Films on Metal Substrates Muhammad Sajid, Asim Khaniya, William Kaden, Abdelkader Kara Molybdenum Nitride exceeds in catalytic activity and product selectivity from industrial grade catalysts for Hydrodenitrogenation (HDN), removal of nitrogen from petroleum feedstocks. δ-MoN is suggested to be more active than bulk γ-Mo2N phase, but more work is needed to better understand the relation between activity and atomic structure of catalysts. We are using DFT with added vdW effects, to study adsorption of both phases on metal substrates, having minimal lattice mismatch. Through Nudged Elastic Band (NEB) Calculations, reaction pathways and energy barriers of HDN reaction mechanisms of pyridine (simplest N containing aromatic molecule) will to studied. Thin films of catalyst phases will be developed under Ultra-High Vacuum (UHV) and studied via spectroscopic techniques (XPS, LEED, ISS, STM) and fixed or variable temperature mass spectrometry. Predictions from theory will be directly compared and verified with experimental observations. Combination from both theory and experiments will aid us towards reaching to the subtle connection between atomic structure and desired catalytic properties. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F52.00009: Spectrally distinctive and highly responsive self-powered MoS2/GaN-nano/Si based photodetector Deependra Singh, Rohit Pant, ARUN CHOWDHURY, Basanta Roul, Karuna Kar Nanda, Saluru Baba Krupanidhi Efficient photodetectors require high and fast photo-response as well as low power consumption. In the present work, we have demonstrated MoS2/GaN nanorods/Si based self-powered and most importantly, spectrally distinctive photodetector. GaN nanorods were grown on Si (111) by Volmer-Weber type growth conditions using plasma-assisted molecular beam epitaxy, followed by growth of MoS2 by pulsed laser deposition. HRTEM analysis shows MoS2 has been incorporated on the top and sides of the nanorods, as well as in between them on Si. The device with top-bottom electrodes shows a maximum responsivity of 10.67 A/W at 900 nm in zero bias condition. The response and the recovery times have been estimated to be 30.9 and 33.9 μs, respectively. The spectral response studies also revealed that in the range of infrared wavelength (1000 - 1050 nm), there is an inversion (positive to negative) in the photocurrent. A mechanism based on illumination modulated activation of barriers, and the interaction of MoS2 with GaN and Si has been discussed to explain the observed phenomenon of polarity inversion. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F52.00010: High-Throughput Experimental Study of Perovskite Ba(ZrYPr)O3-δ (BZYP) Thin Films as Anode Materials for Proton-Conducting Solid Oxide Electrolysis Cells Su Jeong Heo, Meagan Papac, Andriy Zakutayev Proton-conducting solid oxide electrolysis cell (H-SOEC) is of increasing interest as a result of its promising electrochemical application that efficiently converts electrical energy to chemical energy. However, the poor anode activity towards the water-splitting reaction is still the hindrance of the overall performance of H-SOCEs. In this study, novel perovskite Ba(ZrYPr)O3-δ (BZYP) materials are investigated as the anode for the electrolysis cell to improve its proton-conducting ability. We used high-throughput experimental technique to reveal the chemical and electrical properties of BZYP perovskite and identify their appropriate composition for effective water splitting application. The BZYP combinatorial libraries films are synthesized using combinatorial pulsed laser deposition (PLD) and characterized by spatially resolved techniques for composition, structure, thickness, and electrical properties. Overall, through innovative material discovery and characterization using HTS, an anode with extensively broadened reactive sites will be developed for proton-conducting solid oxide electrolysis cells applications. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F52.00011: Optical properties and carrier transport in graphene-mesoporous silicon nanocomposites Défi Junior Jubgang Fandio Graphene outstanding properties has been used to enhance the performance of various optoelectronic devices, in particular the field of photovoltaics [1]. In this work, we investigate the optical properties and charge carrier transport of mesoporous silicon nanocomposites coated with few—layer graphene. Photoconductivity properties and carrier capture/recombination dynamics of the photocarriers in these films are studied via optical pump-terahertz probe measurements. Our results show influence of carbon deposition temperature on the Si nanocrystallites luminescence bands as well as the appearance of new higher energy emission bands. The carrier recombination dynamics is largely affected by the presence of defects at the Si/graphene interface. The frequency dependent photoconductivity curves are well reproduced by the Drude-Smith model, like that used for studying the properties of a film composed of Si nanoparticles dispersed in an oxide matrix [2]. The relatively high mobility and low carrier recombination time in these nanostructures make them good candidates for THz photoswitch applications. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F52.00012: The Role of Metal-Semiconductor Interface in Hybrid Perovskite Devices for High-Performance Solid-State Detectors Shreetu Shrestha, Hsinhan Tsai, Michael Yoho, Fangze Liu, Yusheng Lei, Jon Baldwin, Sergei Tretiak, Ducta Vo, Wanyi Nie Hybrid perovskites have emerged as excellent semiconductors enabling efficient opto-electronic devices. Apart from the intrinsic properties of the semiconductor, interfaces are critical to make a superior device. Here, we use scanning photocurrent microscopy on lateral methylammonium lead triiodide (MAPbI) single crystal devices with commonly used high work function metal and low work function metal contacts to investigate perovskite-metal interfaces. By comparing the spatially resolved photocurrent maps of devices with Au (high work-function metal) to Pb (low work-function metal), we find that a Schottky barrier exists in both cases and the barrier is higher for the Pb/perovskite junction resulting in a lower leakage current. From the decay of the photocurrent profile near the metal contacts, we estimate charge carrier diffusion length to be 9 ± 2 µm. Using this knowledge, we successfully demonstrate a single crystal MAPbI gamma ray detector from which sharp gamma-ray induced pulses are observed. Our study indicates that the interface plays a significant role especially in solid state detector operating at low flux photon counting mode. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F52.00013: p-Diamond as a plasmonic material for high frequency applications Sergey Rudin, Greg Rupper, Tony G. Ivanov, Michael S Shur The gate-controlled hole gas at the hydrogenated diamond surface was predicted to have a plasmonic response to a terahertz and sub-terahertz electric field, making p-diamond field effect transistors promising candidates for implementing room temperature plasmonic devices. The predicted performance of diamond plasmonic detectors shows their potential for high temperature, high voltage, and radiation hard applications and for THz communications and spectroscopy. The hole mobilities in our samples allow for a room temperature resonant plasmonic response. It makes possible the realization of p-diamond based emitters in terahertz and sub-terahertz range, using strong current driven plasma instability in gated channels. Toward the optimal design of p-diamond plasmonic devices we simulated the response using the complete set of hydrodynamic equations, including the thermal transport and accounting for effects of viscosity and pressure gradients. In the strong signal regime we find that a shock wave develops in the charge density and drift velocity profiles. Due to high value of the dielectric breakdown field in diamond the diamond plasmonic device can operate in the strong signal regime, when a shock wave propagates in the channel. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F52.00014: Influence of headgroup and chain length on the surface freezing behavior of long chain amphiphiles Saranshu Singla, Amanda Stefin, He Zhu, Ali N Dhinojwala Long-chain polar amphiphiles such as alcohols and amines form a finite contact angle on high-energy solid surfaces on account of formation of an oriented monolayer, as opposed to expected complete wetting. Previous experiments with octadecanol (and hexadecanol) suggest that the monolayer formed on alumina displays surface freezing behavior on account of strong headgroup-substrate interactions. However, the data for different headgroups and chain lengths is elusive in literature. In the present study, we investigate the combined effect of headgroup-substrate interactions and chain length on the observed surface freezing behavior using interface-sensitive sum frequency generation spectroscopy (SFG). SFG provides information about ordering of amphiphilic molecules at the sapphire-melt interface along with interaction strength of different headgroups with sapphire. Our study provides insights into the fundamental mechanism of monolayer formation. |
Tuesday, March 3, 2020 10:48AM - 11:00AM |
F52.00015: Molecular beam epitaxial growth of high quality BeMgZnO single crystal films for ultraviolet photodetectors Yifei Fang, Longxing Su, Min Xu, Yin Hang High quality ZnO single crystal films and alloy films were prepared on sapphire substrate by molecular beam epitaxy. By studying the influence of growth parameters on quality, components and defect density of the film, repetitive and controllable epitaxial growth conditions were obtained, and the possible positions and effects of doping elements, Be and Mg, in the crystal were further studied. The high quality of film was verified by RHEED and TEM. It is found that with small ionic radius Be doped in the lattice is easy to produce lattice distortion, and has a certain probability of entering the interstitial position. However, Mg atoms have high solid solution in ZnO. Photoconductive, schottky junction and heterojunction UV photodetectors were further prepared and their physical properties were systematically studied. We found that the response speed of the device is greatly slowed down due to the space charge layer caused by O2 adsorption,and the response time is optimized to a scale of microsecond by Be doping. Heterojunction devices have the advantages of self-drive and ultra-fast response, which can be used as self-powered devices in some unmanned and harsh working environments. This research gives guidence for application of ZnO in semiconductor optoelectronic devices. |
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